High-Voltage Batteries

Automotive manufactures have been investigating the use of high-voltage batteries for over 20 years. The first step was to the use of 42-volt systems. Today, high-voltage batteries are required by electric-drive vehicles. This section discussed the high-voltage battery system and its progression.

42-Volt Systems

The first production vehicle that used the 42-volt system was the 2002 Toyota Crown Sedan, which was only sold in the Japanese market. Although the use of 42-volt systems is very limited at the current time, the technology learned has been applied to the electric hybrid systems.

As you have probably come to realize, the increased use of electrical and electronic accessories in today’s vehicles has about tapped the capabilities of the 12-volt/14-volt electrical system. Electronic content in vehicles has been rising at a rate of about 6% per year. It’s been estimated that by the end of the decade the electronic content will be about 40% of the total cost of a high-line vehicle. The electrical demands on the vehicle have risen from about 500 watts in 1970 to about 4,000 watts in 2005. It is estimated that in 10 years the demand may reach 10,000 watts.

Note: The reason for using 42 volts is based on the current 12-volt system. Today’s batteries are rated at 12 volts but actually store about 14 volts. In addition, the charging system produces about 14 to 15 volts when the engine is running. With the engine running, the primary source of electrical power is the charging system. This means that the automobile’s electrical system is actually a 14-volt system. Forty-two volts represent three 12-volt batteries, but since they actually hold a 14-volt charge, the system is considered 42 volts (3 times 14 volts equals 42 volts).

An additional benefit that may be derived from the use of a 42-volt system is it allows manufacturers to electrify most of the inefficient mechanical and hydraulic systems that are currently used. The new technology will allow electromechanical intake and exhaust valve control, active suspension, electrical heating of the catalytic converters, electrically operated coolant and oil pumps, electric air conditioning compressor, brake-by-wire, steer-by-wire, and so on to be utilized. Studies have indicates that as these mechanical systems are replaced, fuel economy will increase by about 10% and emissions will decrease.

Additional fuel savings can also be realized due to the more efficient charging system used for the 42-volt system. Current 14-volt generators have an average efficiency across the engine speed range band of less than 60%. This translates to about 0.5 gallons (1.9L) of fuel for 65 miles (104.6 km) of driving to provide a continuous electrical load of 1,000 watts. With a 42-volt generator, the fuel consumption can be reduced the equivalent of up to 15% in fuel savings.

As simple as it may seem to convert from a 12-volt/14-volt system to a 36-volt/42-volt system, many challenges need to be overcome. It is not as simple as adding a higher-voltage output generator and expecting the existing electrical components to work. One of the biggest hurdles is the light bulb. Current 12-volt lighting filaments can’t handle 42 volts. The dual voltage systems can be used as a step toward full 42-volt implantation. However, dual voltage systems are expensive to design. Another aspect of the 42-volt system is service technician training to address aspects of arcing, safety, and dual-voltage diagnostics. Arcing is perhaps the greatest challenge facing the design and use of the 42-volt system. In fact, some manufacturers have abandoned further research and development of the system because of the problem with arcing. In the conventional 14-volt system, the power level is low enough that it is almost impossible to sustain an arc. Since there isn’t enough electrical energy involved, the arcs collapse quickly and there is less heat build up. Electrical energy in an arc at 42 volts is significantly greater and is sufficient to maintain a steady arc. The arc from a 42-volt system can reach a temperature of 6000°F (3,316°C).

The ISG is usually located between the engine and the transmission in the bell housing

FIGURE. The ISG is usually located between the engine and the transmission in the bell housing.

One of the newest technologies to emerge from the research and development of the 42-volt system is the integrated starter generator (ISG). Although this technology was not actually used on a 42-volt system, development was a result of this system. The ISG is one of the key contributors to the hybrid’s fuel efficiency due to its ability to automatically stop and restart the engine under different operating conditions. A typical hybrid vehicle uses an electric induction motor or ISG between the engine and the transmission.

The ISG performs many functions such as fast, quiet starting, automatic engine stops/starts to conserve fuel, recharges the vehicle batteries, smoothes driveline surges, and provide regenerative braking.

HEV Batteries

As discussed earlier, lead-acid batteries are the most commonly used batteries in the automotive industry. By connecting the batteries in series to each other, they can provide high enough voltages to power some electric vehicles (EVs). For example, the first-generation General Motors’ E V used twenty-six 12-volt lead-acid batteries connected in series to provide 312 volts. The down side of this arrangement is that the battery pack weighed 1,310 pounds (595 kg). In addition, distance that could be traveled between battery recharges was 55 to 95 miles (88 to 153 km). The next-generation EV used nickel-metal hydride (NiMH) batteries. These provided for a slightly longer traveling range between recharges. Although production of battery-powered EVs has slowed, the technology learned has provided for the development of hybrid vehicles. Now the hybrid technology is accelerating battery technology to the point that battery-powered EVs may become more common in the future.

The battery pack in a hybrid vehicle is typically made up of several cylindrical cells or prismatic cells. These battery packs are often called high-voltage (HV) batteries. There are several types of HV batteries being used or in development.

HV battery constructed of cylindrical cells

FIGURE. HV battery constructed of cylindrical cells.

Battery pack made of several prismatic cells

FIGURE. Battery pack made of several prismatic cells.

Nickel-Cadmium (NiCad) Batteries. NiCad cells may have a future role in hybrid vehicles because of several advantages that it has. These include being able to withstand many deep cycles, low cost of production, and long service life. NiCad batteries perform very well when high energy boosts are required.

The negatives associated with the use of NiCad batteries include the following points: they use toxic metals, have low energy density, need to be recharged if they have not been used for a while, and suffer from the memory effect. The memory effect refers to the battery not being able to be fully recharged because it “remembers” its previous charge level. This results in a low battery charge due to a battery that is not completely discharged before it is recharged. For example, if the battery is consistently being recharged after it is only discharged 50%, the battery will eventually only accept and hold a 50% charge and not accept any higher charge.

The cathode (positive) electrode in a NiCad cell is made of fiber mesh covered with nickel hydroxide. The anode (negative) electrode is a fiber mesh that is covered with cadmium. The electrolyte is aqueous potassium hydroxide (KOH). The KOH is a conductor of ions and has little involvement in the chemical reaction process. During discharge, ions travel from the anode, through the KOH, and on to the cathode. During charging, the opposite occurs. Each cell produces 1.2 volts.

Nickel-Metal Hydride (NiMH) Batteries. NiMH batteries are very quickly replacing nickel-cadmium batteries since they are more environmentally friendly. They also have more capacity than the NiCad battery since they have a higher energy density. However, they have a lower current capacity when placed under a heavy load. At this time, the NiMH is the most common HV battery used in the hybrid vehicle.

The issue facing HEV manufactures is that the NiMH battery has a relatively short service life. Service life suffers as a result of the battery being subjected to several deep cycles of charging and discharging over its lifetime. In addition, NiMH cells generate heat while being charged and they require long charge times. Because of the service life issue, most batteries used in HEVs have an eight-year warranty.

The cathode electrode of the NiMH battery is a fiber mesh that contains nickel hydroxide. The anode electrode is made of hydrogen-absorbing metal alloys. The most commonly used alloys are compounds containing two to three of the following metals: titanium, vanadium, zirconium, nickel, cobalt, manganese, and aluminum. The amount of hydrogen that can be accumulated and stored by the alloy is far greater than the actual volume of the alloy.

The cathode and anode electrodes are separated by a sheet of fine fibers saturated with an aqueous and alkaline electrolyte-KOH. The components of the cell are typically placed in a metal housing and then the unit is sealed. There is a safety vent that allows high pressures to escape, if needed.

Under load the cell discharges and the hydrogen moves from the anode to the cathode electrode. Since the electrolyte only supports the ion movement from one electrode to the other, it has no active role in the chemical reaction. This means that the electrolyte level does not change because of the chemical reaction. When the cell is recharged, hydrogen moves from the cathode to the anode electrode.

The cells can be constructed either cylindrical or prismatic. Both designs are currently being used in today’s hybrid vehicles. The prismatic design requires less storage space but had less energy density than the cylindrical design.

A 300-volt battery is constructor of 240 cells that produce 1.2 volts each. The cells are made into a module, with each module having 6 cells. Each module is actually a self-contained 7.2 volt battery. The modules are connected in series to create the total voltage.

Cell module connections

FIGURE. Cell module connections.

Service disconnect plug

FIGURE. Service disconnect plug.

A service disconnect is used to disable the HV system if repairs or service to any part of the system is required. This service connector provides two functions that are used to separate the HV battery pack into two separate batteries, with approximately 150 volts each. First, when the service disconnect is lifted up, it opens a high-voltage interlock loop (HVIL), then when the service disconnect is fully removed, it opens the high-voltage connector. When the HVIL is open, contactors should open. Contactors are heavy-duty relays that are connected to the positive and negative sides of the HV battery. The contactors are normally open and require a 12-volt supply to keep them closed. When the service disconnect is lifted and the HVIL is opened, the voltage supply to the contactors is interrupted and the contactors should open. However, if arcing has occurred that may have welded the contacts of the contactors together, the circuit will not be opened. This will result in a DTC being set.

Lithium-Ion (Li-ion) Batteries. Rechargeable lithium-based batteries are very similar in construction to the nickel-based batteries just discussed. Positives associated with the use of lithium batteries include high energy density, limited memory effect, and they being environmentally friendly. The negatives are lithium is considered an alkali metal and oxidizes very rapidly in air and water, which makes lithium highly flammable and slightly explosive when exposed to air and water. Lithium metal is also corrosive.

Note: Lithium is the lightest metal and provides the highest energy density of all known metals.

The anode electrode of a Li-ion battery is made of graphite (a form of carbon). The cathode mostly comprises graphite and a lithium alloy oxide. Due to the safety issues associated with lithium metal, the Li-ion battery uses a variety of lithium compounds. A manganese li-ion battery has been developed for use in hybrid vehicles that has the potential of lasting twice as long as a NiMH battery.

The electrolyte is a lithium salt mixed in a liquid. Polyethylene membranes are used to separate the plates inside the cells and, in effect, separate the ions from the electrons. The membranes have extremely small pores that allow the ions to move within the cell.

As with most other rechargeable cells, ions move from the anode to the cathode when the cell is providing electrical energy and during recharging, the ions are moved back from the cathode to the anode.

+Lithium-Polymer (Li-Poly) Batteries. The lithium-polymer battery is nearly identical to a li-ion battery and share the same electrode construction. The difference is in the lithium salt electrolyte. The Li-Poly cell holds the electrolyte in a thin solid, polymer composite (polyacrylonitrile) instead as a liquid. The solid polymer electrolyte is not flammable.

The dry polymer electrolyte does not conduct electricity. Instead, it allows ions to move between the anode and cathode. The polymer electrolyte also serves as the separator between the plates. Since the dry electrode has very high resistance, it is unable to provide bursts of current for heavy loads. The efficiency can be increased by increasing the cell temperature above 140°F (60°C). The voltage of a Li-Poly cell is about 4.23 volts when fully charged.